Conventional production techniques for iron, steel and other similar metals involves the pouring of molten metal into an ingot mold in which the metal is chilled to form an ingot which is thereafter further processed. The ingot mold is usually a large cast iron body having an opening therein in which the molten metal is received. Typically, a hot top, of either the combustible or the non-combustible type, is placed upon the mold prior to pouring and provides a reservoir of molten metal which substantially eliminates the problem of pipe formation. Those skilled in the art understand that the chilling of the metal in the relatively cold mold can cause a central void to form in the chilled metal because the specific volume of the metal is greater at the casting temperature than at the chilling temperature.
The hot top serves as a reservoir for molten metal which fills this void and thereby assures that the resulting ingot is substantially void free. It is therefore important that the hot top either provide sufficient heat or prevent heat conductance and thereby loss so that the metal in the reservoir, or the sinkhead as it is known, will not itself chill and therefore be unavailable for preventing pipe formation.
Non-combustible hot tops are designed to be reusable and to provide substantial insulation for the sinkhead in order to prevent premature chilling of the metal in the reservoir. Typically, the hot tops have been large and heavy an thereby cumbersome to use. It is known to move the hot top off the sinkhead after a thin metal shell has formed in order to provide an air gap which provides additional insulation and also prevents the metal from adhering to the hot top.
These conventional non-combustible hot tops may also cause the metal in the sinkhead to be chilled more than is desired because of the mass of the hot top. Naturally, the molten metal in the sinkhead will heat the surrounding walls of the hot top so that heat is therefore lost. Typically, the surrounding hot top may cause the sinkhead to be chilled to a thickness of as much as six times the thickness of the hot top. Those skilled in the art will appreciate that it is desirable to minimize to the maximum extent possible the chilling of the metal in the sinkhead due to heating of the hot top so that more molten metal will be available for preventing pipe formation.
Furthermore, the hot top is typically placed into contact with the cast iron mold by being seated thereon. Both the hot top and the mold are formed of heat absorbing material. Because the hot top is of much less mass than is the mold, there is a natural tendency for the mold to act as a radiator so that the heat in the sinkhead is drained away to the mold and thereby the chilling increased as the mold is heated.